1. Field of Invention
The present invention relates to the field of microelectronic integrated circuits. Specifically, the present invention relates to simultaneous formation of the top oxide in a nonvolatile silicon oxide nitride oxide silicon (SONOS) based nonvolatile memory transistor and a gate oxide for a metal oxide semiconductor (MOS) transistor included within control circuits.
2. The Relevant Technology
Some semiconductor chips include SONOS based nonvolatile memory transistors, and MOS transistors in various configurations. In particular, the MOS transistors are located within control circuits, which can be located in the nonvolatile memory array, volatile memory array, or a peripheral control circuit. The MOS transistors in the control circuits include, in part, pass-gate transistors in the nonvolatile memory array for controlling access to SONOS based nonvolatile memory transistors, peripheral MOS transistors in the peripheral control circuit, and volatile transistors in the volatile memory array.
A current process flow for fabricating a nonvolatile memory device separately forms the top oxide in a oxide-nitride-oxide (ONO) structure and a gate oxide of a MOS transistor used in a control circuit. Specifically, the tunnel oxide, nitride, and the top oxide are sequentially formed on a silicon substrate for the ONO structure. The top oxide layer is typically a deposited oxide which is not dense and easily etched during subsequent cleaning processes. As such, the top oxide layer may be further densified to minimize losses to the top oxide layer in further processing steps.
The ONO layers are patterned and etched from the silicon surface except for areas where the SONOS memory transistors are formed. Also, the gate oxides of MOS transistors located on the semiconductor chip are formed separately through thermal oxidation. Unfortunately, even after densification, the top oxide of the ONO structure is susceptible to etching during subsequent cleaning steps, which results in a non-uniform top oxide layer of the ONO structure. As a result, the thickness of the top oxide layer in the ONO structure is hard to control, which can lead to lower yields.